Self-testing of symmetric three-qubit states
Pith reviewed 2026-05-24 21:43 UTC · model grok-4.3
The pith
Symmetric three-qubit states that superpose the W and GHZ states can be self-tested device-independently from their measurement statistics.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We propose self-testing schemes for a large family of symmetric three-qubit states, namely the superposition of W state and GHZ state. We first propose and analytically prove a self-testing criterion for the special symmetric state with equal coefficients of the canonical basis, by designing subsystem self-testing of partially and maximally entangled state simultaneously. Then we demonstrate for the general case, the states can be self-tested numerically by the swap method combining semi-definite programming (SDP) in high precision.
What carries the argument
Analytic self-testing criterion obtained by simultaneous subsystem self-testing of a partially entangled two-qubit state and a maximally entangled two-qubit state (equal-coefficient case); swap method inside semidefinite programming (general case).
If this is right
- The equal-coefficient symmetric state admits a fully analytic self-testing proof.
- Arbitrary superpositions of W and GHZ admit high-precision numerical self-testing.
- Self-testing now covers a continuous family of symmetric three-qubit states rather than isolated examples such as pure Dicke or graph states.
- Certification requires only the number of measurements, the number of outputs, and the observed statistics.
Where Pith is reading between the lines
- Device-independent protocols that consume these states as resources could be certified without trusting the state-preparation devices.
- The same combination of analytic subsystem arguments and numerical SDP may extend self-testing to other continuous families of symmetric multipartite states.
- Exact analytic criteria found for special coefficient values can serve as benchmarks for checking the accuracy of numerical SDP relaxations on nearby states.
Load-bearing premise
The observed statistics are assumed to arise exactly from the claimed projective measurements performed on the target symmetric state.
What would settle it
Exhibiting a different three-qubit state (or a higher-dimensional state) together with measurements that produce exactly the same joint probability tables would falsify the uniqueness claim.
Figures
read the original abstract
Self-testing refers to a device-independent way to uniquely identify the state and the measurement for uncharacterized quantum devices. The only information required comprises the number of measurements, the number of outputs of each measurement, and the statistics of each measurement. Earlier results on self-testing of multipartite state were restricted either to Dicke states or graph states. In this paper, we propose self-testing schemes for a large family of symmetric three-qubit states, namely the superposition of W state and GHZ state. We first propose and analytically prove a self-testing criterion for the special symmetric state with equal coefficients of the canonical basis, by designing subsystem self-testing of partially and maximally entangled state simultaneously. Then we demonstrate for the general case, the states can be self-tested numerically by the swap method combining semi-definite programming (SDP) in high precision.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes self-testing schemes for a large family of symmetric three-qubit states, namely superpositions of the W state and GHZ state. It analytically proves a self-testing criterion for the special case with equal coefficients by simultaneous subsystem self-testing of partially and maximally entangled states. For general coefficients, it demonstrates self-testing numerically via the swap method combined with semi-definite programming (SDP) in high precision.
Significance. The analytic proof for the equal-coefficient special case is a strength, as it supplies a rigorous uniqueness result grounded in standard quantum mechanics and self-testing definitions without free parameters or fitted quantities. This extends self-testing beyond the previously considered Dicke and graph states. The numerical SDP results for the general family provide supporting evidence of practical utility but do not rise to the level of a uniqueness proof.
major comments (1)
- [Abstract] Abstract: The numerical demonstration for general coefficients via the swap method and SDP in high precision does not establish rigorous uniqueness. SDP relaxations are outer approximations to the set of quantum correlations, and floating-point results are never exactly tight; therefore high-precision numerics alone cannot exclude the existence of other states or measurements that reproduce the observed statistics within the reported tolerance. This concern is load-bearing for the central claim that covers the large family of states.
Simulated Author's Rebuttal
We thank the referee for the careful reading and constructive feedback. We appreciate the positive assessment of the analytic proof for the equal-coefficient case. We agree that the numerical SDP results constitute supporting evidence rather than a rigorous uniqueness proof, and will revise the manuscript to clarify this distinction in the abstract and discussion.
read point-by-point responses
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Referee: [Abstract] Abstract: The numerical demonstration for general coefficients via the swap method and SDP in high precision does not establish rigorous uniqueness. SDP relaxations are outer approximations to the set of quantum correlations, and floating-point results are never exactly tight; therefore high-precision numerics alone cannot exclude the existence of other states or measurements that reproduce the observed statistics within the reported tolerance. This concern is load-bearing for the central claim that covers the large family of states.
Authors: We agree with the referee. The SDP-based numerical demonstration supplies high-precision evidence but cannot rigorously establish uniqueness, as correctly noted regarding relaxations and floating-point precision. In the revised manuscript we will update the abstract to state that the general family is supported by numerical evidence via the swap method and SDP (rather than claiming it establishes self-testing with the same rigor as the analytic case), and we will add a brief discussion of this limitation in the conclusions. revision: yes
Circularity Check
No circularity: analytical construction and numerical SDP are independent of target state by definition.
full rationale
The manuscript derives its self-testing criterion for the equal-coefficient case via an explicit analytical construction of simultaneous subsystem self-testing on partially and maximally entangled states; this is a direct proof from observed correlations and does not reduce to the target state by construction or by fitting. The general-coefficient case is handled by the standard swap-method SDP relaxation, which is an external numerical verification tool rather than a self-referential fit or self-citation. No load-bearing self-citation, ansatz smuggling, or renaming of known results appears; the derivation chain remains self-contained against standard quantum-information benchmarks.
Axiom & Free-Parameter Ledger
axioms (1)
- standard math Quantum mechanics with projective measurements and the operational definition of self-testing from statistics alone.
Reference graph
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with different kinds of coefficients. A. Self-testing of a symmetric three-qubit state The specific case we consider is the state with equal coefficient of the basis, reads as: |ψ ⟩ = 1√ 5 (|001⟩ + |010⟩ + |100⟩ + |000⟩ + |111⟩). (9) The basic idea of self testing this state is to project the state onto two kinds of subsystem entangled states by one FIG. 1. The...
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and ( 48), where Γ is a matrix with NPA hierarchy characterization of the quantum behaviors. This moment matrix corre- sponding to q-local level 1 (which includes any products with at most one operator per party) has size 74 × 74 7 and is augmented by necessary terms(like ⟨ZAXAZA⟩, ⟨ZBXBZCXC⟩, ⟨ZCXCZAXAZA⟩ and so on) to express all the average values ⟨·⟩t...
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0 0 √ 1 6 (3 − √ 5) (56) is diagonal, U and V are unitary matrices: U = 3+ √ 5 2 √ 5+2 √ 5 3− √ 5 2 √ 5− 2 √ 5 1+ √ 5 2 √ 5+2 √ 5 1− √ 5 2 √ 5− 2 √ 5 , V = 1+ √ 5 2 √ 5+ √ 5 1− √ 5√ 10− 2 √ 5√ 2 5+ √ 5 5+ √ 5 10 . (57) Define |j′ A⟩ ≡ ∑ j Uji |j⟩, |k′ B⟩ ≡ ∑ k Vik |k⟩ and λ i = Sii, for i,j,k ∈ { 0, 1}. Obviously, {|j′⟩}A and {|k′⟩}B are ...
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